There is a shock absorber piston known from EP-A-0 658 611 that has a piston body that is provided with a peripheral web at its peripheral surface. A number of webs running in the axial direction attach to this peripheral web on one side of it. A seal made of thermoplastic synthetic is sprayed onto this piston body in an injection molding process. The webs extending in the axial direction and the notches between them, which are filled in with sealing material, serve to reliably anchor the sealing material. The seal applied by injection molding enables close tolerancing, which prevents xe2x80x9cblow-byxe2x80x9d and thus provides a reliable seal of the cylindrical spaces facing one another. The process to produce these types of injection-molded seals is relatively expensive.
There is a piston-cylinder arrangement known from U.S. Pat. No. 3,212,411 whose piston body has a number of peripheral grooves on its peripheral surface. To apply the seal, a cup-shaped preliminary mold made of PTFE (polytetrafluoroethylene) is provided that is first placed on the piston body loosely. The piston body so prepared is then pressed into a forming and calibrating cylinder that is heated to a high temperature. Under the influence of the heat, the PTFE material is pressed into the grooves on the peripheral surface of the piston body. Then, the piston body with the pressed-on seal is cooled in an appropriately designed cooling cylinder. The grooves are completely filled with the sealant material so as to provide a form-locked solid connection of the seal to the peripheral surface of the piston body. When used as a shock absorber piston, the bottom surface of the preliminary mold that still overlaps the end surface of the piston body on one side must then be removed.
There is a shock absorber piston known from EP-A-682 190 whose only essential difference from the processes described above in its manufacture is in that to apply the seal, instead of a cup-shaped preliminary mold, a stamped circular sleeve is used. This circular sleeve is placed on one end of the piston body. The piston body prepared thusly is then pressed into a heated forming and calibrating cylinder, wherein the circular sleeve is placed around the peripheral surface of the piston body as a strip and then pressed into the grooves running in the peripheral direction of the piston body under the influence of heat. Then, the piston with its pressed-on seal is guided through a cooling tube. Here, as well, the sealant material fills the grooves practically completely so that the seal is solidly connected to the peripheral surface of the piston body in form-locked fashion.
The two processes described above have the disadvantage in that considerable pressures are required to shape and to press the sealant material into the grooves on the peripheral surface of the piston body. Also, the sealant material forming the seal is subject to strong shaping forces that disadvantageously influence the structure of the sealant material. Only through exact calibration can the desired degree of seal be reached. Differences in the change in diameter in the piston and cylinder due to temperature influences cannot be compensated any more than seal wear can be compensated.
The objective of this invention is to create a piston, in particular a shock absorber piston, in which the disadvantages described above are prevented.
This objective is met according to the invention by a piston for a piston-cylinder arrangement, in particular a shock absorber piston, with a piston body that is provided with at least two peripheral webs on its peripheral surface that border a notch. Furthermore, in this piston arrangement, a collar-shaped seal made of a thermoformable plastic material is formed onto the webs and overlap them. There are also axial flow channels in the piston body that are closed by means of throttle valves that open on one side only, as well as at least one radial opening that connects at least one flow channel to the notch between two webs. Surprisingly, it has been shown that, in order to get an acceptable and reliable connection between the seal and the piston body, it is not necessary to arrange a multitude of grooves on the peripheral surface of the piston body. A minimum of two peripheral webs bordering a notch are sufficient here, onto which the collar-shaped seal is formed in the manner described in EP-A-682-190. It has also been shown, surprisingly, that it is sufficient for the web to press into the material of the seal only along a portion of its height. On one hand, this results in an acceptable form-lock between the collar-shaped seal and the piston body, and on the other hand, only moderate shaping forces result on the plastic material so that not only do less pressure forces have to be applied, but material flow is also kept to a very minimum during the deformation, thus preventing a disadvantageous influence on the material structure for practical purposes. The collar-shaped seal overlapping the two webs results in a small cavity between the inside surface of the seal and the notch, said cavity encircling the piston body in the peripheral direction. Since the flow channels in the piston body are closed in alternating fashion by the throttle valves that open on one side, a portion of the flow channels are closed by the throttle valves in one direction of motion due to the pressure building up in the cylinder, whereas the throttle valves of the other flow channels open so that the hydraulic fluid can flow through from one cylinder space to the other cylinder space. In the process, a relatively high fluid pressure builds up in the open flow channels. Since at least one of these flow channels is provided with a radial opening that opens up into the annular space between the collar-shaped seal and the notch base, a portion of the hydraulic fluid can enter into this annular space and an corresponding pressure can build up here. Since the collar-shaped seal is made of a plastic material and can be elastically deformed within certain limits, and whereas the annular gap between the outer peripheral surface of the collar-shaped seal on one side and the associated cylinder wall on the other is very small, the collar-shaped seal bulges outward into the annular space under the influence of the pressure, pressing against the cylinder wall. Since the pressure in the flow channel depends on the load on the shock absorber, the seal of the piston in the cylinder depends necessarily on the load.
Depending on which flow channel is chosen to have the radial opening, it is possible to selectively provide this pressure-dependent change in pressure force of the seal for a motion of the piston in one or the other direction.
In another advantageous embodiment of the invention, it is provided that each of the edges of the collar-shaped seal extends beyond the end surface of the piston body associated with it. Since the collar-shaped seal is produced according to a known process from a circular sleeve, the phenomenon of xe2x80x9cback memoryxe2x80x9d of the sealant material described in EP-A-0 682 190 can be used to cause the edge around the inner diameter of the circular sleeve to pull inward after it is applied to the piston body, and to cause the edge of the collar-shaped seal produced from the external edge of the circular sleeve to move back outward and in this way to protrude above the rest of the peripheral surface of the collar-shaped seal as a lip seal. If the piston body is installed such that, when used as a shock absorber piston, the piston surface provided with the lip-shaped edge extending outward faces the pressurized side, i.e. the side subjected to the high load, and if the piston surface with the edge that springs back inward is located on the so-called suction side, this results in an improved seal of the piston in the shock absorber cylinder during a pressure load since the hydraulic fluid in the lip-shaped edge presses against the cylinder wall.
When it springs back, i.e. for suction loads, the hydraulic fluid can then enter the intermediate space between the piston wall and the cylinder wall to some degree as a result of the minimal play between the two. The fluid can thus find its way up to the edge where the lip-shaped edge of the collar-shaped seal sits against the cylinder wall for the purposes of lubrication. Thus, acceptable lubrication is provided, preventing wear of the seal. Since the flow resistance due to this minimum gap between the sealing surface of the piston and the cylinder wall is considerably higher than the flow resistance through the flow channels in the piston body, practically no drop in performance results.
The reduction of sealing effect of the collar-shaped seal described above during spring-back can now be smoothed out with the concept according to the invention by assigning the radial opening in the piston body to at least one of the flow channels that has hydraulic fluid flowing through it against the action of the throttle valve during spring-back, i.e. when under suction. As a result of the elastic deformation of the collar-shaped seal caused by the pressure build-up in the annular space, this motion causes the seal to be pressed against the cylinder wall, thus compensating for the reduced seal effect caused by the lack of the lip seal.
In an especially advantageous embodiment of the invention, it is provided that the webs are each located in the area adjacent to the end surface of the piston body. This geometry purposefully provides only two webs, which then sit on both sides of only one notch in the peripheral surface of the piston body lying in between. The webs can be dimensioned to be relatively wide; the notch laterally bordered by the webs can then be twice to three times as wide as the width of a web. This simplifies the contour of the peripheral surface of the piston body considerably.
In an especially advantageous embodiment of the invention, the piston body is pieced together from at least two element sections, each of which has a plane of separation that runs perpendicular to the piston axis. This type of piston body design is suited for manufacture using a powder-metallurgy process, i.e. as a sintered metal part. The form of the two element sections is particularly favorable if each element section has only one peripheral web so that the mirror-imaged element sections pieced together border a notch with their webs. This allows the manufacture of an [undercut] notch.
In a variation of the invention, furthermore, two collar-shaped seals are arranged next to one another on a piston body with two webs, each of these seals covering a web. The edges of the seals extending beyond the end surfaces of the piston body can be formed to face in and/or out and the other edges of the two seals can be formed to lie adjacent to one another near the notch between the two webs. Since the sealing function under load is essentially a result of the edges of the seal acting as a lip seal, the hydraulic fluid flowing in from a flow channel through the radial opening into the notch can, by appropriate minimal dimensioning the radial opening, flow through the gap between the two collar-shaped seals and at the same time lubricate the sealing surfaces between the cylinder wall and the collar-shaped seal. Since the radial opening itself acts as a throttle, only a minimal pressure build-up results in the area of the notch so that the sealing effect of the lip seal on the respective pressure side is not affected. The lip seal located on the low pressure side can however xe2x80x9cfloat on topxe2x80x9d of the hydraulic fluid flowing out of the gap between the two collar-shaped seals resulting in lubrication.